I am currently using a NAS for sharing storage accross all my Macs. Since I do not need concurrent collaboration, I stored my Aperture library inside a sparsebundle image disk on the NAS.

Whoever needs to work with the library first mounts the sparsebundle and open the library directly in Aperture. Note that storing the library inside a sparsebundle image disk is necessary for Aperture since it cannot delete files directly on a network drive, it needs a drive that supports the Mac trash, so a sparsebundle image disk.

If you need concurrent work, you will need multiple Aperture libraries, one per user. Each libray can then reference images that can be located in a central location, again a NAS server or equivalent. Same as before, you cannot store referenced images directly on a network drive as Aperture will not be able to delete them so you need to use a sparsebundle image disk if you need to manage your files directly from Aperture.

This last workflow will improve performance and responsiveness in Aperture since the library will be stored locally but each user will need to do some work whenever you add new photos to the central location.